Computational Study of Molecular Hydrogen Adsorption over Small (MO₂)_<i>n</i> Nanoclusters (M = Ti, Zr, Hf; <i>n</i> = 1 to 4)

Hydrogen adsorption on small group 4 metal oxide clusters for both the singlet and the first excited triplet states have been investigated by density functional theory and correlated molecular orbital theory at the coupled cluster CCSD­(T) level. The reaction starts with hydrogen physisorption on a metal center followed by formation of metal hydride/hydroxides due to splitting H₂ into H^– and H⁺. The hydrogen physisorption energies are predicted to be −1 to −8 kcal/mol for the singlet and −1 to −26 kcal/mol for the triplet, respectively. The formation of metal hydride/hydroxides does not involve redox processes. Chemisorption leading to formation of metal hydride/hydroxides is exothermic by −10 to −50 kcal/mol for the singlet, and exothermic by up to −60 kcal/mol for the triplet. The predicted energy barriers are less than 20 kcal/mol. Formation of metal dihydroxides from the metal hydride/hydroxides is generally endothermic for the monomer and dimer and is exothermic for the trimer and tetramer. Formation of the dihydroxide is a proton coupled electron transfer (PCET) process. The singlet energy barriers for the H^–→ H⁺ transfer process are predicted to be 35–60 kcal/mol, in comparison to triplet energy barriers of less than 15 kcal/mol for the H^• → H⁺ transfer process. For trimers and tetramers, there exist two different pathways: the first is a direct pathway with PCET to a terminal oxygen and the second is a two-step pathway with initial formation of a bridge OH group followed by a proton transfer to generate a terminal OH group. For the singlet, the two-step pathway is preferred for M = Ti and the direct pathway is more favorable for M = Zr and Hf. The two-step pathway is always preferred for the triplet as one-electron transfer is always more likely than two-electron transfer in the direct pathway